Wash and cut the squash into pieces (you can keep the skin). If you are adding a potato, clean it, peel it and cut it into large chunks.

Chop the shallots.

Heat a kettle of water.

In a large saucepan or casserole, melt half the butter and fry the squash, optional potato, and shallots for about 3/4 minutes.

Then pour in the boiling water until it covers the vegetables, add the stock cubes and mix well. Cover with a lid and let it cook over a low-medium heat.

In the meantime, chop the chestnuts very coarsely and fry them in the remaining butter in a pan over a low heat for about 5 minutes.

After 15 minutes of cooking the soup, add 2/3 of the chestnuts and let it cook for about another 5 minutes. Check with a knife that the flesh of the squash is tender and stop cooking.

Remove 5 ladles of stock and reserve it (you can use it for another recipe or add more stock later if the soup is too thick).

Blend the soup. Add a little stock if the consistency is too thick.

Taste, then add the sour cream, nutmeg, a pinch of salt and pepper. Taste again and adjust the seasoning. Just before serving, pour the soup into bowls or soup plates and sprinkle with the remaining crumbled chestnuts.